The last thing either the government or the Metropolitan police wanted, on the day that Britain played host to the G20 leaders last week, was a death during the demonstrations being staged simultaneously in the City of London. So perhaps it should be no surprise that initially the fate of Ian Tomlinson, the man who died in the midst of the main protest close to the Bank of England, was barely noted.

Although the Guardian reported the death on its front page, almost all the coverage elsewhere ignored it completely or concentrated on a version of events that suggested that the police's only connection with Tomlinson had been to try to rescue him from a baying mob of anarchists. The police were "pelted with bottles by a screaming mob" (the Mirror) or "pelted with bottles as a medical team tried to revive a demonstrator" (Mail). Tomlinson had died "after being 'caught among the mob'" (Telegraph). The BBC TV night-time news the following day made no mention of the death in its main bulletin.

The general overview of the demonstrations in the media was either one of mockery of the protesters or the implication that the City had had a fortuitous escape from complete anarchy. The Sun reported that "foaming at the mouth and smelling of stale cider, packs of protesters lurch(ed) through the city". An occasional commentator was wheeled on to say that the police had not used tear gas or water cannon, as they would almost certainly have done in other countries. The implicit suggestion was that the protesters should be grateful that the authorities in Britain are not like, say, the neo-fascist thugs of the Genoa police who methodically battered defenceless protesters in the wake of the 2001 anti-globalisation protests in Italy. Certainly there were many good-natured police officers on the ground who tried to defuse the situation, and who were as baffled as anyone by their superiors' rigid tactics of containment, or "kettling", that caused so much confusion and tension on the day. At the same time, as many, many witnesses have reported, there were other officers hyped up for a ruckus who behaved, particularly at the Climate Camp in nearby Bishopsgate, after the cameras had departed, with the same sort of random, out of control, violence as that attributed to protesters.

Most of last week's demonstrators were not born when Kevin Gately died a few miles away in Red Lion Square in 1974. A young student from Warwickshire university, he was taking part in a demonstration against the National Front. Lord Scarman conducted an inquiry into what had happened but reached no conclusions. Five years later, schoolteacher and activist Blair Peach died in another anti-racist protest in Southall. In neither case, despite public inquiries, was the truth of what took place ever officially established. One of the problems in both those high-profile cases was that witnesses' versions of events differed dramatically and there was little in the way of objective evidence to prove what many of the demonstrators believed had happened.

We live in very different times now. One of the striking aspects of the 1 April demonstration was that, wherever you turned, someone seemed to be pointing a camera. The police were videoing from rooftops and windows, their spotters pointing out suspects. The protesters were cheerfully taking souvenir shots of themselves with mobile phones on the steps of the Bank of England. The media were there in numbers. The local CCTV cameras are also, it appears, always with us.

Some of the miles and miles of footage that was shot has now been given to the Guardian and shared across the internet. It shows Tomlinson, who was not a demonstrator but one of the many people unable to leave the melee, being thrown to the ground a few moments before he died of a heart attack. Far from the police coming under attack, at this stage Tomlinson is only cared for by a demonstrator. Do the police have their own film of what happened?

What is also striking is that, so soon after the inquest into the death of the Brazilian Jean Charles de Menezes, assumptions about a suspicious death should be so swiftly made and the official version accepted so unquestioningly. One of the Met's major problems in the wake of de Menezes was the feeling that misinformation about the circumstances of his death was allowed to linger too long in the public domain.

Of course, the police are under pressure to come up with instant information for the ever-increasing media outlets. A man has died. How? Why? Who was he? It is hardly suprising that the police's best take on the incident – that they were the subject of attack by demonstrators as they tried to save a man's life – is the one that gets passed out and then gets prime position in the coverage. But when did it become clear to the police, from their own intelligence and video footage, what had actually happened to Tomlinson?

The two lessons must be that, as always, we should never assume that the first official version of a death in suspicious circumstances is accurate. The second lesson must be that the police have now to review their tactics for future demonstrations.

A man with a weak heart died. Was he prevented from leaving a scene of mayhem, of police, mounted and in riot gear, of barking dogs and bonfires? We were meant to recall the G20 summit as the start of a new world order. It may now turn out to be a rather less glorious view of the mechanics of law and order.